Don't you notice that everything that happened with Helen & Nikki involved Shell.

Episode 1. Nikki gets the fashion show cancelled after Carol Byatt loses her baby. Shell gets mad becuase she can't show off her new "cozzie".

Shell tells Fenner that Nikki told on him about him to Marilyn which results in Nikki getting shipped out.

Shell creates a bogus Diary that makes Helen look incompetent.

Shell uses Nikki's name as Naughty Nikki when she performs as an erotic dancer in Amsterdam.

Shell gets upset because Fenner is no longer Wing Governor when Helen comes back at the beginning of Series 2.

I always thought that Shell was Nikki's doppleganger, the evil version of Nikki. Like, if someone just heard about Nikki (eg lesbian cop killer etc) they'd think of someone like Shell: deranged, violent etc. Similarly, when Helen gets anxious about her feelings for Nikki, and the impropriety of them, what she's worried about is that she's like Fenner, and her relationship with Nikki is like Fenner's with Shell: full of exploitation and manipulation. So, yeah, I think there's a ton of mirroring between Nikki and Shell, and between Helen-Nikki and Fenner-Shell.

It's interesting to think that Shell's breakdown mid-season 2 actually serves as a precursor or foreshadowing of Nikki's jealousy and rage a bit later in the season. And then of course Nikki's breakout in the season 2 finale and Shell's attack on Fenner aren't dissimilar either--both are acts of liberation in a way, and of course Shell's attack is a penetrative sexual act, although not the kind any of us would want to participate in. :)